Tissue stimulators have gained wide acceptance in the field of medicine for the treatment of chronic, intractable pain. Generally, tissue stimulators include electrical circuits for generating electrical stimulation pulses, electrodes for attaching to the affected part of the body, and leads for conveying the stimulation pulses from the generating circuits to the electrodes. In some cases the entire tissue stimulator system is intended to be implanted within the body while in other cases the pulse generating circuitry is contained in a package externally of the body. In the case of transcutaneous stimulators, electrodes having a significant surface area are held in contact with the skin by adhesives or other means over the affected areas. Another type of stimulation which can be used either with external or implanted pulse generators uses leads extending to an implanted electrode, for example one implanted along the spinal cord. In any case, the application of the electrical stimulation pulses to body tissue produces the effect of relieving or masking the sensation of pain. In more sophisticated units, controls or programming may be provided to adjust parameters of the output stimulation such as pulse amplitude and repetition rate. Many tissue stimulator pulse generators also provide a burst or cycle mode wherein groups of individual stimulating pulses are provided at intervals, with delay intervals between the groups.
It is very desirable to provide some means of control over the amplitude or intensity of the stimulating pulses so that the patient using the device can adjust the amplitude for maximum effectiveness. If the amplitude is too low, there may be insufficient relief; and if the amplitude is too high, there can be an unpleasant stinging or tingling sensation. The optimum amplitude changes according to a variety of operating conditions including the length of time the stimulation has been on. Problems have been encountered in burst or cycle mode stimulation wherein the unpleasant stinging or tingling sensation sometimes accompanies the stimulation.
Circuits have been devised in the prior art for providing a gradual increase in the amplitude of pulses within a group of pulses in a burst mode to alleviate this problem. Circuits have utilized resistance-capacitance charging or discharging characteristics to provide a gradual increase in amplitude of the first several pulses within a group leading up to the final pulse amplitude. While this technique has helped in alleviating unwanted tingling sensation, it is subject to the disadvantage of lack of adaptability to different increase rates or slopes and different final pulse amplitudes. Further, resistance-capacitance charging circuits can lead to somewhat higher electrical consumption which may be a major consideration in the case of fully self contained, implantable pulse generator.